How environmentally friendly is a product? Life cycle assessments aim to provide an answer. Now PSI researchers show how to make such assessments more precise and more complete.
Image Credit: © Paul Scherrer Institute PSI/AI-generated symbolic image
Scientific Frontline: Extended "At a Glance" Summary: Hybrid Life Cycle Assessment
The Core Concept: Life cycle assessment (LCA) is a mathematical method used to quantify the environmental impact of products and services by analyzing their supply chains. Recent advancements emphasize hybrid life cycle assessments (HLCA), which merge conventional process data with macroeconomic trade data to eliminate data gaps and provide a complete environmental footprint.
Key Distinction/Mechanism: Conventional life cycle assessments are inherently subjective because analysts must arbitrarily define where a product's supply chain boundary ends. Hybrid assessments overcome this limitation by utilizing global input-output tables, mathematically integrating the complete economic interconnectivity of all sectors to measure environmental impacts without arbitrary cutoffs.
Origin/History: While conventional LCA procedures are standardized by the International Organization for Standardization (ISO), researchers have been developing hybrid methodologies since the 1970s. In 2026, researchers at the Paul Scherrer Institute and the CIRAIG Life Cycle Assessment Research Center demonstrated that the four historically recognized hybrid methods can be mathematically unified into just two primary approaches.
Major Frameworks/Components:
- Conventional process assessment: The traditional method of logging specific manufacturing and supply data (e.g., electricity consumption or raw material use), which remains vulnerable to subjective boundary limits.
- Input-output tables: Comprehensive economic datasets that map financial expenditures and associated emissions between global industrial sectors.
- Tiered HLCA: A simpler hybrid method that supplements direct process data with broader economic sector averages, ideal for assessing products with low macroeconomic impact (e.g., luxury watches).
- Integrated HLCA: A complex, bidirectional method that continuously feeds data between economic tables and process chains. This requires sector reclassification and is necessary for analyzing systemic, large-scale economic shifts (e.g., expanding nationwide wind energy).
Branches of Science: Industrial Ecology, Environmental Economics, Energy Systems Analysis, and Applied Mathematics.
Future Application: These unified mathematical models are laying the groundwork for accessible, open-source software (based on existing platforms like Brightway) that will allow users to conduct complex hybrid life cycle assessments without requiring advanced mathematical expertise.
Why It Matters: Accurate environmental impact measurements are essential for establishing objective climate policies, guiding corporate sustainability strategies, and fairly enforcing international trade frameworks like the European Carbon Border Adjustment Mechanism, which directly influences consumer costs.
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| “A life cycle assessment is ultimately a mathematical method: it is only as good as the data it is based on,” says Michael Weinold. He was able to show that four known methods for hybrid life cycle assessments can be mathematically reduced to two. Photo Credit: © Vanessa Döringer/ecoinvent |
Life cycle assessment is used to quantify the environmental impact of products and services and forms an important basis for political decisions. However, the necessary data is always difficult to capture completely. A study by the Paul Scherrer Institute (PSI) now shows that four established hybrid methods can be reduced to just two, marking a first step toward open-source software that could enable such analyses even without in-depth mathematical knowledge.
Determining the comprehensive environmental impact of products—ranging from vehicles to wind turbines—typically relies on life cycle assessment (LCA). This method captures the upstream supply chain of a product or service. “Life cycle assessment is ultimately a mathematical method; it is only as good as the data it is based on,” says Michael Weinold, who now works at ETH Zurich. He carried out the research at PSI’s Laboratory for Energy Systems Analysis.
A primary limitation of conventional life cycle assessments is their inability to capture all processes contributing to a product's environmental impact. Where the boundary lines are drawn is decided by those who create the life cycle assessment. Weinold closely examined methods that can close these gaps using data from the global economy. During this analysis, a methodological inconsistency emerged: while several “hybrid” life cycle assessment methods existed, the mathematical differences between them remained poorly defined.
Together with Guillaume Majeau-Bettez, a professor at the CIRAIG Life Cycle Assessment Research Center in Canada, Weinold investigated the various methods. In two studies published in the Journal of Industrial Ecology, the researchers demonstrate that four established hybrid methods can be mathematically reduced to just two. They also provide recommendations on which method should be used and when.
Boundary Truncation and System Limitations A life cycle assessment is supposed to capture data from a product’s entire life cycle—a process chain that includes not only manufacturing but also raw materials, energy supply, transportation, and many other activities. For example, when it comes to gasoline for a car, it’s not just the combustion that matters, but also the origin of the fuel. “For crude oil, this chain usually ends where the oil is extracted,” says Weinold. “Theoretically, however, research, administration, and real estate projects of the oil companies could also be included.”
Where should the line be drawn? The decision depends on those carrying out the analysis. Weinold noted that this introduces a subjective element into life cycle assessments. “And this is despite the fact that the procedure is standardized by ISO—where the system boundaries lie is not specified in the standard.”
This subjective component can have direct effects on the economy, for example, with the new European Carbon Border Adjustment Mechanism. This scheme applies a carbon price to imported goods based on their specific carbon footprint. This will, for example, make importing steel from China more expensive. “This is of great importance to the economy,” Weinold says. “The calculations must be as objective as possible, because ultimately, we all pay the costs as consumers.”
Combining Detailed Knowledge and Completeness The solution is combining detailed data on production processes with global economic data. This information is captured in so-called input-output tables. They show how economic sectors are interconnected and what the resulting environmental impacts are. The tables show, for example, how much money the US aviation industry spends on steel. If the \(\text{CO}_2\) emissions from steel production are also known, it is possible to calculate how much of this can be attributed to aviation.
Methods that combine classic process data with input-output tables for the economy are called hybrid life cycle assessments (HLCAs). “The economic tables are, by definition, complete,” says Weinold. “Therefore, with hybrid life cycle assessments, there’s no need to draw an arbitrary line anymore.”
Mathematical Unification of Hybrid Methods Since the 1970s, researchers have been developing various methods for creating hybrid life cycle assessments. “So far, four main methods and several variants thereof have been scientifically recognized,” Weinold explains. In a complex research project, he analyzed the different procedures and ultimately was able to show that all of these methods can be mathematically reduced to two.
The authors of one of the four main methods were initially skeptical and requested additional calculations for various special cases. In the end, it turned out that the differently formulated methods are in agreement mathematically.
The two remaining methods differ in their complexity. In the simpler “tiered HLCA” approach, the classic process assessment is supplemented with data from input-output tables. For example, in the life cycle assessment of cheese, specific data such as milk quantity or electricity consumption are used. Where there is a lack of such data, average data from the dairy sector is used. “This method is suitable for products whose production volumes are small in the overall economic context, such as Swiss watches,” Weinold says. “For processes that are relevant to the overall economy, the second method must be applied.”
Bidirectional Data Integration in HLCA In this so-called integrated HLCA approach, data can flow not only from the economic table into the process chain but also back again, which necessitates a reclassification of economic sectors.
Weinold explains what this means using the example of the expansion of wind energy in Switzerland: a pilot project with ten wind turbines has little impact on the nationwide energy system—a case for the tiered hybrid life cycle assessment. However, analyzing a scenario in which 20 percent of Swiss electricity production is supplied by wind power requires accounting for macroeconomic consumption shifts. “Then the integrated method is needed,” Weinold says.
Now, Weinold’s findings are to be put into practice: the goal is to develop software for creating hybrid life cycle assessments that can be used even without in-depth mathematical knowledge. The model is Brightway, PSI-developed open-source software for traditional life cycle assessments, which is now one of the most widely used tools in this field.
Published in journal: Journal of Industrial Ecology (1&2)
Title:
- Mathematical unification of methods for hybrid life cycle assessment
- Critical analysis of the path-exchange method for hybrid life cycle assessment
Authors:
- Michael P. Weinold, and Guillaume Majeau-Bettez
- Michael P. Weinold, and Guillaume Majeau-Bettez
Source/Credit: Paul Scherrer Institute | Barbara Vonarburg
Edited by: Scientific Frontline
Reference Number: eco111026_01
